The Security Mentor

Apple released a bunch of security fixes. One of them's really important. Without the fix, you could lose control of your machine just by opening a TIFF graphics file from the wrong source.

The only way this could be worse would be if there were bad guys using it to take over computers. I haven't heard of any. Yet.

Go to System Preferences, the item on the Dock that looks like a light switch, and ask for a software update. Your system will remind you soon anyway but I wouldn't wait if I were you.